This invention relates in general to power control systems and in particular to a control system and method for voltage stabilization in an electric power system.
Time varying loads can result in unwanted voltage fluctuations in a power supply network. An example of such a load are alternating current (AC) electric arc furnaces, which are commonly used to melt and remelt ferrous materials such as steel, and to smelt non-ferrous materials. Such furnaces generally use high power arcs to generate heat energy in a refractory lined vessel, and include a power supply for controlling the electrical energy supplied to the arc. High power arcs are an energy conversion mechanism that behave as a non-linear time-varying impedance. Consequently, the voltage, current and power drawn by an arc furnace tends to fluctuate, causing disturbances to both the melting process and to the supply network. These disturbances can result in inefficiencies, increased equipment wear, disturbances to the power network, and in extreme cases damage to the supply network or arc furnace. The voltage disturbances that occur in the supply network arising from large and rapid fluctuations in the load current and power factor during certain operating stages of the furnace are often referred to as xe2x80x9cflickerxe2x80x9d. Furnace flicker is a common problem for both furnace operators and power distributers. Power distributers will often place strict limits on flicker caused by furnaces that draw power from their distribution systems in order to reduce disturbances to such distribution systems.
Various technologies have been developed for power control and flicker reduction for arc furnaces. One commonly used technology that has been used on steel electric arc furnaces is the static VAR compensator (SVC). An SVC consists of a shunt connected harmonic filter bank and a shunt connected thyristor-controlled reactor, which operate in concert to lower voltage flicker or maintain a constant furnace power factor. The SVC operates by shunt injection of either capacitive or inductive reactive power, thereby maintaining a constant voltage by maintaining the total reactive power draw (MVAR) of the furnace balanced near zero (ie. neither inductive or capacitive). SVC""s typically have a half cycle time delay due to thyristor commutation requirements. An example of an early SVC can be seen in U.S. Pat. No. 3,936,727.
SVC based arc furnace controllers dynamically supply reactive power by the controlled summation of constant capacitive MVAR and variable inductive MVAR. The controller compares load reactive power to a set point power factor and dynamically controls the summated MVAR to the set point. As an electric arc furnace frequently short circuits and open circuits on bore in of the furnace electrodes, the furnace reactive power swings vary from zero to 200% of the furnace transformer rating. An SVC is normally sized at 125% to 150% of the furnace rating and typically reduces flicker by approximately 40% to 50%. Some SVCs use a voltage set point, and adjust a shunt reactor to match a process voltage to the set-point voltage.
A variation of the SVC technology which has been developed relatively recently is known as STATCOM (Static Synchronous Compensator) or SVC Light, which consists of parallel connected insulated gate bipolar transistors (IGBT) powered by a DC capacitor voltage source. Response speed is a function of the IGBT switching frequency and the coupling reactor size.
Another flicker reduction technology is the smart predictive line controller (SPLC) that consists of a thyristor connected in series with the arc electrode and a harmonic filter bank. An SPLC functions as a dynamically controlled series reactor that uses predictive software to stabilize the current on a electric arc furnace. The SPLC reduces flicker by lowering arc:current fluctuations on the power systems. When arc current fluctuations are flat lined, the voltage flicker is reduced. An example of an SPLC can be seen in U.S. Pat. No. 5,991,327 issued Nov. 23, 1999.
Although existing technologies such SPLC""s and SVC""s have been used to mitigate voltage flicker to some extent, it is desirable to have a power control system that provided improved flicker regulation beyond that presently available.
According to the present invention, variable shunt and series connected reactors are used in a complimentary combination to provide improved flicker and power control for a time-varying load such as an arc furnace. According to another aspect, a series connected inductive reactor is varied either to stabilize current or real power draw depending on flicker levels.
According to one aspect of the invention, there is provided a power control system for an AC time-varying load connected to an AC power supply line. The power control system includes a first variable inductive reactor intermediate the power supply line and the load, a second variable reactor connected in parallel with the power supply line, and a control system for (i) monitoring load current and adjusting the first variable inductive reactor in response to changes in the monitored load current to reduce voltage flicker; and (ii) monitoring reactive power draw from the AC power supply line and adjusting the second variable reactor in response to changes in the monitored reactive power draw to reduce voltage flicker.
According to another aspect of the invention, there is provided a power control system for an AC electric arc furnace having an AC power supply line for applying power to an electrode. The system includes a variable inductive series reactor connected intermediate the power supply line and the electrode, a variable inductive parallel reactor connected in parallel with the power supply line, a harmonic capacitive filter bank connected in parallel with the power supply line, and a control system for mitigating voltage flicker on the power supply line. The control system includes (i) current stabilizing means for adjusting the inductive series reactor to stabilize an electrode current to control the voltage flicker; (ii) reactive power stabilizing means for adjusting the inductive parallel reactor to stabilize a reactive power draw from the AC power supply line to control the voltage flicker; and (iii) control means for monitoring voltage flicker on the power supply line and adjusting operating parameters of the current stabilizing means and the reactive power stabilizing means based on the monitored voltage flicker.
According to another aspect of the invention, there is provided a method for controlling voltage flicker in an AC power supply line having a time-varying load connected thereto, including (a) providing a variable inductive series reactor intermediate the power supply line an d the load; (b) providing a variable parallel reactor in parallel with the power supply line; (c) varying an inductance of the variable inductive series reactor to reduce voltage flicker; and (d) varying a reactance the variable parallel reactor to reduce voltage flicker. Preferably, in step (c) the inductance of the variable inductance series reactor is varied to reduce voltage flicker occurring primarily within a selected first frequency range, and in step (d) the reactance of the variable parallel reactor is varied to reduce voltage flicker occurring primarily within a selected second frequency range.
According to still a further aspect of the invention, there is provided a power control system for an AC power supply line having a time-varying load connected thereto, including a variable inductive reactor intermediate the power supply line and the load, a flicker meter for monitoring voltage flicker on the AC power supply line, a current stabilizer for varying the variable inductive reactor to stabilize a current draw from power supply line to reduce voltage flicker on the power supply line when the monitored voltage flicker is above a first threshold, and a real power stabilizer for varying the variable inductive reactor to stabilize a real power draw from the power supply line when the monitored voltage flicker is below a second threshold.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.